1. Field of the Invention
The present invention is directed to anti-reflective coatings, methods for making the anti-reflective coatings, and products prepared by the methods.
2. Background
Optical reflections occur when light passes from one medium to a second medium when the refractive index (“n”) of the two media differs. Thus when light passes from air (n=1) to glass (n=1.5) there are reflections. Reflections can be minimized or eliminated by gradually grading the index of refraction from a first material to a second material. In theory, for a single-layer coating on glass in air, the optimum material has a refractive index of n=1.23. While most layered coatings can exhibit small differences in the refractive index between materials or between layers of a coating, multi-layer films can reduce this problem. However, few solid materials are known having a refractive index, n<1.2; and few robust materials are known having a refractive index, n<1.3. Thus, all presently known thin layer coatings suffer from an abrupt decrease in refractive index at the coating-air interface that gives rise to reflection of electromagnetic radiation from the top-most surface of the substrate.
One method to minimize reflection from a surface is to include single- and multi-layer thin films that incorporate destructive interference. However, destructive interference does not work well for incoming light not perpendicular to the surface.
A second theoretical solution to providing anti-reflection is to provide a porous nanostructured laminar Gradient Refractive Index (“laminar GRIN”) coating in which the porosity of a coating material is controlled on the nanometer scale to achieve refractive index values from n=1.0 to about n=1.4. For example, a glass material having a refractive index, n=1.5, that is made 80% porous will have a refractive index, n=1.1, so long as the length scale of the porosity within the porous glass material is small enough to avoid light scattering. In an ideal system, the refractive index would be controllably decreased from n=1.5 to n=1.0 by controllably increasing the volume fraction of air within the material. However, ideal laminar GRIN structures have yet to be made using thin, solid films.
The natural world is also replete with examples of anti-reflective structures. For example, the surface of a moth eye is covered with domes having a height and radius of about 150 nm to 250 nm, which provide excellent antireflective properties across the visible spectrum.
The fabrication of nanostructured films that are similar to an ideal theoretical or a naturally occurring anti-reflective coating has proved exceedingly difficult. For example, top-down manufacturing of GRIN, “moth-eye” structures have been demonstrated with only limited efficacy, largely because standard lithography processes either lack the necessary resolution or are ill-suited for creating vertically tailored structures. Holographic lithography has proven more versatile at creating GRIN structures, but these techniques are expensive and still limited in their resolution, which decreases the bandwidth of the anti-reflective coatings.
A second approach to fabricating GRIN nanostructures is by growth methods. For example, porous nanowire films, porous glass films, and porous polymer films can be prepared by depositing a binary mixture and subsequently removing one component. However, the formation of a film with varying porosity has proven difficult to control, while the formation of multi-layer structures with decreasing porosity typically suffers from collapse due to high porosity and low mechanical integrity of the outer layer(s) of the film.